Method for producing anhydrous formic acid

ABSTRACT

The invention relates to a process for obtaining anhydrous or substantially anhydrous formic acid, in which firstly aqueous formic acid is produced by hydrolysis of methyl formate, with the methanol content in the methyl formate having been reduced in advance. The process according to the invention has the special feature that—before the hydrolysis of the methyl formate—the methanol content of the methanol-containing methyl formate is reduced in a distillation column, this distillation column simultaneously being employed for other separation functions during work-up of the formic acid.

The present invention relates to a process and an apparatus forobtaining anhydrous or substantially anhydrous formic acid.

“Ullmanns Encyklopädie der technischen Chemie” [Ullmann's Encyclopediaof Industrial Chemistry], 4^(th) Edition, Volume 7, page 365, disclosesthat formic acid can be prepared by acidolysis of formamide usingsulfuric acid. However, this process has the disadvantage thatstoichiometric amounts of ammonium sulfate are obtained as anunavoidable product.

Another way of preparing formic acid consists in the hydrolysis ofmethyl formate, which is synthesized from methanol and carbon monoxide.This synthesis is based on the following equations:$o\quad v\quad e\quad r\quad a\quad l\quad{l:\begin{matrix}\left. {{C\quad O} + {C\quad H_{3}{—OH}}}\quad\rightarrow\quad{C\quad H_{3}{—O—CO—H}} \right. \\\left. {{C\quad H_{3}{—O—CO—H}} + {H_{2}O}}\quad\rightarrow\quad{{C\quad H_{3}{—OH}} + {H\quad{—CO—OH}}} \right. \\\left. {{C\quad O} + {H_{2}O}}\quad\rightarrow\quad{H\quad{—CO—}\quad O\quad H} \right.\end{matrix}}$

The hydrolysis of methyl formate described in “Ullmanns Encyklopädie dertechnischen Chemie” [Ullmann's Encyclopedia of Industrial Chemistry],4^(th) Edition, Volume 7, page 366HCOOCH₃+H₂O≈HCOOH+CH₃OHhas the disadvantage of an unfavorable position of the hydrolysisequilibrium. A shift in the equilibrium by removing the desired processproducts by distillation is not possible since methyl formate (boilingpoint 32° C.) boils significantly lower than methanol (boiling point 65°C.) and formic acid (boiling point 101° C.). Anhydrous formic acidcannot easily be obtained from the resultant aqueous formic acidsolution by distillation since it forms an azeotrope with water. Thedifficulty thus consists in obtaining anhydrous formic acid from themethyl formate hydrolysis mixture.

A process described in EP-B-0 017 866 which comprises the importantsteps a) to g) enables the preparation of anhydrous formic acid startingfrom methyl formate. Anhydrous formic acid is obtained here if

a) methyl formate is subjected to hydrolysis,

b) methanol and excess methyl formate are distilled off from theresultant hydrolysis mixture,

c) the bottom product from the distillation (b), which comprises formicacid and water, is extracted in a liquid-liquid extraction with anextractant which principally takes up the formic acid,

d) the resultant extract phase, comprising formic acid, extractant andsome of the water, is subjected to distillation,

e) the top product obtained in this distillation, which comprises waterand some of the formic acid, is fed back into the lower part of thedistillation column in step (b),

f) the bottom product from distillation step (d), which comprisespredominantly extractant and formic acid, is separated by distillationinto anhydrous formic acid and the extractant, and

g) the extractant leaving step (f) is fed back into the process.

In this process, it is particularly advantageous

h) to carry out distillation steps (b) and (d) in a single column,

i) to introduce the water necessary for the hydrolysis in the form ofsteam into the lower part of the column provided for carrying out step(b),

k) to employ methyl formate and water in the hydrolysis (a) in a molarratio of from 1:2 to 1:10, and/or

l) to employ, as extractant, a carboxamide of the general formula I

-   -   where the radicals R¹ and R² are alkyl, cycloalkyl, aryl or        aralkyl groups, or R¹ and R² jointly, together with the N atom,        form a heterocyclic 5- or 6-membered ring, and only one of the        radicals is an aryl group, and where R³ is hydrogen or a        C₁-C₄-alkyl group.

Steps (a) to (i) are explained in greater detail below.

Step (a)

The hydrolysis is usually carried out at a temperature in the range from80 to 150° C.

Step (b)

The distillation of the hydrolysis mixture can in principle be carriedout at any desired pressure, preferably from 0.5 to 2 bar. In general,working under atmospheric pressure is advisable. In this case, thetemperature at the bottom of the column is about 110° C. and thetemperature at the top of the column is from about 30 to 40° C. Thehydrolysis mixture is advantageously added at a temperature in the rangefrom 80 to 150° C., and the methanol is preferably removed in liquidform at a temperature of from 55 to 65° C. Satisfactory separation ofthe mixture into methyl formate and methanol on the one hand and aqueousformic acid on the other hand is possible even using a distillationcolumn which has 25 theoretical plates (the theoretical number of platesis preferably from 35 to 45). Any design can be used for the columnintended for step (b), but a sieve-plate or packed column isparticularly recommended.

Step (c)

The liquid-liquid extraction of the formic acid from its aqueoussolution by means of an extractant is preferably carried out atatmospheric pressure and a temperature of from 60 to 120° C., inparticular from 70 to 90° C., in countercurrent. Depending on the typeof extractant, extraction devices having from 1 to 12 theoreticalseparation stages are generally required. Suitable extraction devicesfor this purpose are in particular liquid-liquid extraction columns. Inmost cases, satisfactory results are achieved using from 4 to 6theoretical separation stages.

The choice of extractant is not limited. Particularly suitableextractants are carboxamides of the general formula I given above.Extractants of this type are, in particular, N,N-di-n-butylformamide andin addition N,N-di-n-butylacetamide, N-methyl-N-2-heptylformamide,N-n-butyl-N-2-ethylhexylformamide, N-n-butyl-N-cyclohexylformamide andN-ethylformanilide, and mixtures of these compounds. Further suitableextractants are, inter alia, diisopropyl ether, methyl isobutyl ketone,ethyl acetate, tributyl phosphate and butanediol formate.

Step (d)

The extract phase is separated by distillation in an appropriatedistillation device into a liquid phase, which generally comprisespredominantly formic acid and extractant, and a vapor phasepredominantly comprising water and small amounts of formic acid. This isan extractive distillation. The bottom temperature is preferably from140 to 180° C. A satisfactory separation effect is generally achievedfrom 5 theoretical plates.

Step (e)

The formic acid/water mixture is generally recycled in vapor form.

Steps (f) and (g)

The distillation device (usually in the form of a column) for carryingout step (f) is advantageously operated under reduced pressure—fromabout 50 to 300 mbar and correspondingly low head temperatures—fromabout 30 to 60° C.

Step (h)

This variant of the process relates to steps (b) and (d). Thedistillation devices for carrying out steps (b) and (d) are arranged inan overall distillation device. The distillation devices here aregenerally in the form of columns.

Step (i)

In this step, water required for the hydrolysis is provided in the formof steam.

The industrial synthesis of methyl formate from carbon monoxide andmethanol is carried out using a molar excess of methanol. The reactordischarge from the corresponding synthesis reactor therefore, therefore,in addition to methyl formate, still contains considerable quantities ofexcess methanol. The reactor discharge usually contains from 20 to 40%by weight of methanol—the remainder consisting essentially of methylformate. The economic efficiency of the process described in EP-B-0 017866 is improved by reducing the content of methanol in the mixture ofmethyl formate and methanol taken from the synthesis reactor and onlythen feeding the mixture to the hydrolysis reactor for carrying out stepa). The most economical procedure is to introduce a methyl formatestream comprising about 95% of methyl formate into the hydrolysisreactor—the methanol content in this stream is accordingly significantlylower than in the stream leaving the synthesis reactor. The reduction inthe methanol content is carried out in a distillation column fitted witha rectifying section and having a return at the top. The evaporationenthalpy of the return in this column must be introduced in the stillevaporator in the form of steam, i.e. additional energy must beintroduced into the process. Besides this disadvantage of high energycosts, the economic efficiency of the process is also adversely affectedby the high investment costs for the corresponding column. The costs forthe provision of methyl formate containing only small quantities ofmethanol (for example 5% by weight) thus reduces the economic advantagearising from the use of highly concentrated methyl formate.

It is an object of the present invention to provide a process in whichanhydrous or substantially anhydrous formic acid is obtained. The aim isto improve the economic efficiency of the process in such a way thatexpenditure on energy and equipment for reducing the content of methanolin the mixture of methyl formate and methanol employed for thehydrolysis is reduced.

We have found that this object is achieved by a process for obtaininganhydrous or substantially anhydrous formic acid in which

-   -   i) methyl formate is subjected to hydrolysis,    -   ii) methanol and excess methyl formate are distilled off from        the resultant hydrolysis mixture,    -   iii) the bottom product from distillation ii), comprising formic        acid and water, is extracted in a liquid-liquid extraction with        an extractant which principally takes up the formic acid,    -   iv) the resultant extract phase, comprising formic acid,        extractant and some of the water, is subjected to distillation,    -   v) the top product obtained in this distillation, which        comprises water and some of the formic acid, is fed back into        the lower part of the distillation device in step ii),    -   vi) the bottom product from distillation step iv), which        comprises predominantly extractant and formic acid, is separated        by distillation into anhydrous or substantially anhydrous formic        acid and the extractant, and    -   vii) the extractant leaving step vi) is fed back into the        process,        which comprises feeding methanol-containing methyl formate into        the distillation device proposed for carrying out step ii)        before the hydrolysis of the methyl formate (step i)), where the        corresponding feed point for the methanol-containing methyl        formate in the distillation device is located above the removal        point for methanol and below the removal point for methyl        formate, and methyl formate with a reduced methanol content is        obtained at the removal point for methyl formate and is        subsequently fed to step i).

The term “substantially anhydrous formic acid” is taken to mean formicacid which comprises up to a maximum of 30% by weight, preferably up toa maximum of 15% by weight, of water. The term “methyl formate with areduced methanol content” is taken to mean methyl formate which containsless methanol than the methyl formate leaving the synthesis reactor.

An essential advantage is that methyl formate having a higher content ofmethanol (for example 30%) can also be provided for the processaccording to the invention. This results in a reduction in expenditureon energy and equipment, and the process can thus be operated moreeconomically. An essential advantage is thus that a simple distillationcolumn in the form of a stripping column (without rectifying section andreturn) is sufficient for concentrating the methyl formate from thesynthesis reactor if the partially concentrated methyl formate streamobtained in this way is fed into the distillation device ii), where itcan be concentrated to the desired 95%, for example, without additionalenergy expenditure.

In a preferred embodiment of the invention, the methanol-containingmethyl formate is obtained by feeding the mixture of methyl formate andmethanol removed from the synthesis reactor to a distillation column,preferably in the form of a stripping column, removing some of themethanol therein, and removing the methanol-containing methyl formate atthe upper end of the distillation column. In general, the distillationcolumn, preferably in the form of a stripping column, has no rectifyingsection. There is preferably no return in the upper section of thedistillation column. A distillation column of this type is able toreduce the methanol content in the methyl formate, which is subsequentlyfed to the distillation device for carrying out step ii), to asufficient extent. A particular advantage of a distillation column ofthis type is the lower construction costs, which can be minimized, inparticular, through the omission of a rectifying section and a return.Due to the reduction in the amount of heating steam, the size of thecolumn's heat exchangers and the cooling water needed by thecorresponding condenser can also be reduced.

Distillation steps ii) and iv) are advantageously carried out in asingle distillation device. This correspondingly carries out thefunctions of the individual distillation columns. The water needed forthe hydrolysis (step i)) is preferably fed in the form of steam to thelower part of the distillation device proposed for carrying out stepii).

An apparatus for carrying out the process described above is alsoprovided in accordance with the invention. This comprises

α) a synthesis reactor,

β) a hydrolysis reactor,

χ) a distillation device for carrying out step ii), having a feed pointfor methanol-containing methyl formate and removal points for methanoland methyl formate,

δ) a distillation device for carrying out step iv),

ε) an extraction device, and

φ) a distillation device for carrying out step vi).

Suitable synthesis reactors are all reactors which are suitable for thepreparation of methyl formate starting from methanol and carbonmonoxide. The distillation devices χ), δ) and φ are generally in theform of columns. The extraction device employed is preferably aliquid-liquid extraction column.

In a preferred embodiment, the apparatus according to the invention alsocomprises, besides elements α) to φ), a distillation column, which ispreferably in the form of a stripping column, i.e. has neither arectifying section in the upper part nor is provided with a return. Thedistillation column preferably in the form of a stripping column servesfor the removal of excess methanol from the reaction mixture in thesynthesis reactor.

The distillation device for carrying out step ii) and the distillationdevice for carrying out step iv) are preferably arranged in a singledistillation device. The latter is usually in the form of a large columnhaving the distillation device for carrying out step ii) in the upperpart and the distillation device for carrying out step iv) in the lowerpart.

The drawing shows

in FIG. 1 a diagram of a plant for the preparation of anhydrous orsubstantially anhydrous formic acid in accordance with the prior art,

in FIG. 2 a diagram of a plant for the preparation of anhydrous orsubstantially anhydrous formic acid in accordance with the prior art,where the distillation device for carrying out step ii) and thedistillation device for carrying out step iv) are arranged in a singledistillation device,

in FIG. 3 a diagram of a plant for carrying out the process according tothe invention for the preparation of anhydrous or substantiallyanhydrous formic acid, and

in FIG. 4 a diagram of a plant for carrying out the process according tothe invention for the preparation of anhydrous or substantiallyanhydrous formic acid in which the distillation device for carrying outstep ii) and the distillation device for carrying out step iv) arearranged in a single distillation device.

The reference numerals entered above or alongside the arrows indicatethe components which generally have a high content or the principalcontent in the respective streams. Since the proportions of thecomponents in the streams may vary, these reference numerals should onlyserve as guide values for information. Reference numeral 21 denotesmethyl formate, 22 denotes water, 23 denotes formic acid, 24 denotesmethanol, 25 denotes extractants, 26 denotes offgas and 27 denotescarbon monoxide. It is common to the prior-art process and the processaccording to the invention that methyl formate is prepared in asynthesis reactor 6, the hydrolysis of the methyl formate is carried outin a hydrolysis reactor 1, and step ii) is carried out in thedistillation device 2, the extraction is carried out in an extractiondevice 3, step iv) is carried out in a distillation device 4, and stepvi) is carried out in a distillation device 5.

In FIG. 2 and FIG. 4, the distillation devices 2; 4 are arranged in asingle distillation device 7. The processes according to the inventionand the prior-art processes differ in that the methyl formate- andmethanol-containing stream discharged from the synthesis reactor is fedto distillation columns 8 with different designs. The distillationcolumns in accordance with the prior art contain a rectifying section 11and a return 10—these elements are not present in the plants for theprocess according to the invention. The methyl formate containing aresidual amount of methanol leaving the distillation column 8 is, in theprior-art process, fed directly into the hydrolysis reactor 1. Bycontrast, the methanol-containing methyl formate leaving thedistillation column 8 is, in the process according to the invention, fedto the distillation device 2, the corresponding feed point 9 beingarranged above the removal point for methanol 12 and below the removalpoint for methyl formate 13. The methyl formate obtained at the removalpoint 13 for methyl formate has had its methanol content reduced in thedistillation device 2. This methyl formate with a reduced methanolcontent is then fed to the hydrolysis reactor 1.

The invention will be explained in greater detail below with referenceto a working example.

COMPARATIVE EXAMPLE

The comparative example corresponds to a process in accordance with theprior art which is carried out in a plant shown diagrammatically inFIG. 1. 5.3 kg of aqueous formic acid are prepared. The mixture ofmethyl formate and methanol leaving the synthesis reactor 6 is fed intothe distillation column 8 containing the rectifying section 11, methylformate being concentrated to 95% by weight in this column.

The latter is fed into the hydrolysis reactor 1. The reaction productfrom the synthesis reactor contains 25% by weight of methanol inaddition to the methyl formate. The distillation column 8 having therectifying section 11 is in the form of a bubble-cap plate, with a totalof 30 plates. The mixture removed from the synthesis reactor 6 isintroduced onto plate 15. The bubble-cap plate column contains a stillevaporator which supplies 0.9 kW of heat, and a head condenser whichwithdraws 0.95 kW of heat. The distillation device 2 is in the form of aglass bubble-cap column with 80 plates. The feed of the mixture removedfrom the hydrolysis reactor 1 takes place onto plate 15. In addition,the condensed, liquid head stream from the distillation device 4 isintroduced into the still of the distillation device 2. Liquid methanolfor recirculation into the methyl formate synthesis is taken off at theremoval point 12—the removal point 12 is located at plate 50. 95% byweight methyl formate is taken off at the removal point 13 at the top ofthe distillation device 2. To this end, 4.9 kW of heat must be suppliedin the still evaporator of the corresponding distillation device 7. 4.5kW of heat are withdrawn at the head condenser.

EXAMPLE

The following illustrative experiment corresponds to a process accordingto the invention carried out in a plant shown diagrammatically in FIG.3. As in the above prior-art process, 5.3 kg of aqueous formic acid areprepared. The reaction product from synthesis reactor 6 contains 25% byweight of methanol in addition to methyl formate. The concentration ofthe methyl formate is carried out in a distillation column 8 in the formof a “pure” stripping column which has only 15 plates. The mixtureremoved from the synthesis reactor is introduced onto the uppermostplate, and consequently the column has no rectifying section 11. 0.6 kWof heat—33% less than in the corresponding prior-art process—isintroduced into the still evaporator of the distillation column 8, and0.73 kW −23% less than in the prior-art process—of heat is withdrawn atthe corresponding condenser (the saving of cooling water is associatedtherewith). With the aid of this apparatus arrangement, methyl formateis concentrated to 89% by weight. The resultant stream is introducedinto the distillation device 2 at the feed point 9. The distillationdevice 2 differs with respect to the one employed in the comparativeexperiment only through the feed point 9. This is located at plate 65.The energy introduced by the still evaporator of the distillation device7 corresponds to that in the comparative experiment. The methyl formateconcentration at the removal point 13 is 95% by weight.

The distillation column 8 in the form of a “pure” stripping column hasonly half the physical height compared with the distillation column 8employed in the comparative experiment. Owing to the low energyintroduction necessary, the heat exchangers (evaporator/condenser) canbe 30% smaller. The column diameter can be reduced by 10%.

It can clearly be seen that the process according to the invention issignificantly improved over the prior-art process with respect to energyand investment costs.

1. A process for obtaining anhydrous or substantially anhydrous formicacid, in which i) methyl formate is subjected to hydrolysis, ii)methanol and excess methyl formate are distilled off from the resultanthydrolysis mixture, iii) the bottom product from distillation ii), whichcomprises formic acid and water, is extracted in a liquid—liquidextraction with an extractant which principally takes up the formicacid, iv) the resultant extract phase, comprising formic acid,extractant and some of the water, is subjected to distillation, v) thetop product obtained in this distillation, which comprises water andsome of the formic acid, is fed back into the lower part of thedistillation device in step ii), vi) the bottom product fromdistillation step iv), which comprises predominantly extractant andformic acid, is separated by distillation into anhydrous orsubstantially anhydrous formic acid and the extractant, and vii) theextractant leaving step vi) is fed back into the process, whichcomprises feeding methanol-containing methyl formate into thedistillation device for carrying out step ii) before the hydrolysis ofthe methyl formate (step i)), where the corresponding feed point for themethanol-containing methyl formate in the distillation device is locatedabove the removal point for methanol an below the removal point formethyl formate, and methyl format with a reduced methanol content isobtained at the removal point for methyl formate and is subsequently fedto step i).
 2. The process as claimed in claim 1, wherein themethanol-containing methyl formate is obtained by feeding the mixture ofmethyl formate and methanol removed from the synthesis reactor to adistillation column, separating off some of the methanol therein, andremoving the methanol-containing methyl formate at the upper end of thedistillation column.
 3. The process as claimed in claim 2, wherein thedistillation column has no rectifying section.
 4. The process as claimedin claim 2, wherein the distillation column has no return in the uppersection.
 5. The process as claimed in claim 1, wherein distillationsteps ii) and iv) are carried out in single distillation device.
 6. Theprocess as claimed in claim 2, wherein the distillation column to whichthe mixture of methyl formate and methanol removed from the synthesisreactor is fed is a stripping column.